This invention relates to a frequency error detecting device for detecting a frequency error between a carrier frequency and a standard frequency in a quadrature demodulator.
A quadrature demodulator is used for quadrature demodulating a received signal into a demodulated signal. The received signal is transmitted as a transmitted signal from a transmission section through a transmission path which may be a radio channel. The transmitted signal is derived by modulating a carrier signal at a frame period by a data signal. As a result, the transmitted signal has a sequence of symbols at a predetermined symbol rate. The carrier signal has a carrier frequency. In order to quadrature demodulate the received signal into the demodulated signal, it is generally necessary to regenerate the carrier signal as a regenerated carrier signal from the demodulated signal in the quadrature demodulator.
On the other hand, another quadrature demodulator is known as a conventional quadrature demodulator which quadrature demodulates the received signal into the demodulated signal without use of the regenerated carrier signal. The conventional quadrature demodulator quadrature demodulates the received signal into the demodulated signal by the use of a standard signal having a standard frequency. The conventional quadrature demodulator is disclosed in Japanese examined publication Tokkai Hei 3-128550, namely, 128550/1991.
In order to make the standard frequency be coincident with the carrier frequency, it is necessary to preliminarily detecting a frequency error between the carrier frequency and the standard frequency as a detected frequency error. Namely, it is necessary to preliminarily compensate the standard frequency by the use of the detected frequency error.
On compensating the standard frequency, a frequency error detecting signal is used as the received signal. The frequency error detecting signal is defined by a sequence of signal vectors among which a phase variation is equal to a predetermined phase variation. As a result, the demodulated signal is defined by a sequence of demodulated vectors.
A conventional frequency error detecting device is supplied with the demodulated signal as a first signal. The conventional frequency error detecting device is further supplied with a second signal which is obtained by quadrature demodulating the received signal on the basis of the carrier signal. The conventional frequency error detecting device calculates the detected frequency error in accordance with the first and the second signals.
By the way, the conventional frequency error detecting device comprises an analog-digital (A/D) converter for sampling the demodulated signal at a predetermined sampling rate to quantize the demodulated signal into a quantized signal. The A/D converter generally has an offset frequency. As a result, the quantized signal may include the offset frequency. The conventional frequency error detecting device further comprises a signal processor for calculating the detected frequency error on the basis of the quantized signal and the second signal.
Furthermore, the received signal is inevitably subjected to a noise when transmitted through the radio channel. As a result, the demodulated signal is accompanied by a noise signal.
As readily understood from the above description, it is difficult to correctly detect the frequency error in the conventional frequency error detecting device by existence of the offset frequency and the noise signal.